Mineral Species: Orpiment and Realgar

Chemical Formula: As2S3

Chemical Name: Arsenic Trisulfide

Mineral Class: Sulphide

Mineral Notes: Orpiment is a rather rare mineral, but is nevertheless an important ore of Arsenic. Like Cinnabar and Stibnite, it is a late magmatic mineral, associated with low temperature hydrothermal springs. It is commonly formed by alteration of other Arsenic minerals which decompose on exposure to light. Its yelow colour contrasts stongly with the closely related red-coloured Realgar. The only other common yellow mineral found in similar environments is Sulfur, which lacks Orpiment's perfect cleavage flexible cleavage flakes.

Description: A sizeable specimen of mostly massive Orpiment, characteristically yellow in colour. In many places on the surface mica-like crystals can be seen, as well as larger blocky aggregates. The luster is very strong, many of the flat crystals being partially transparent. In several places the deep red colouration of the closely related Realgar is easily noticed.

Location: Potosi District, Humboldt County, Nevada, USA.